Depends on the internal pressure whether he became more or less buoyant, and the unknown (to me at least) is whether a fart is more or less dense than air at the same pressure. He did lose mass, but then we get into the semantics of whether “heavier” was really used to mean actual weight, or just the downward force exerted on the ground. The other question is, if he gained more downward force on his feet by releasing the hot gas, would the increase in the frictional force on his feet improve his athletic ability enough to make up for the loss of momentum used for tackling due to the mass loss, and what role would the discomfort of having a bolus of gas trapped in your rectum play in your ability to perform.